[Solved] Couldn’t match type Synonym with Either

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Either isn’t simply a union of types; it’s a tagged union, which means every value has to explicitly specify which “side” of the type the wrapped value occurs on. Here’s an example, (with a Show instance derived for your Card type):

*Main> Card Hearts Jack

<interactive>:3:13: error:
    • Couldn't match type ‘Court’ with ‘Either Pip Court’
      Expected type: Rank
        Actual type: Court
    • In the second argument of ‘Card’, namely ‘Jack’
      In the expression: Card Hearts Jack
      In an equation for ‘it’: it = Card Hearts Jack

Card expects an argument of type Either Pip Court, but you gave it a plain Court value. With an explicit wrap:

*Main> Card Hearts (Right Jack)
Card Hearts (Right Jack)

The same applies to creating a numbered card:

*Main> Card Hearts 3

<interactive>:5:13: error:
    • No instance for (Num Rank) arising from the literal ‘3’
    • In the second argument of ‘Card’, namely ‘3’
      In the expression: Card Hearts 3
      In an equation for ‘it’: it = Card Hearts 3
*Main> Card Hearts (Left 3)
Card Hearts (Left 3)

You don’t need to change the definition of snap, because there is already an Eq instance for Either as long as both wrapped types have Eq instances themselves; it considers any Right value unequal to any Left value, and matching Right or Left values are equal if the wrapped values are equal.

*Main> snap (Card Hearts (Right Jack)) (Card Hearts (Left 3))
"..."
*Main> snap (Card Hearts (Right Jack)) (Card Spades (Right Jack))
"SNAP"

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solved Couldn’t match type Synonym with Either